pinctrl: Document pinctrl-single,pins when #pinctrl-cells = 2

pinctrl: Document pinctrl-single,pins when #pinctrl-cells = 2

Document the values in pinctrl-single,pins when #pinctrl-cells = <2>

Fixes: 27c90e5e48d0 ("ARM: dts: am33xx-l4: change #pinctrl-cells from 1 to 2")
Reported-by: Trent Piepho <tpiepho@gmail.com>
Signed-off-by: Drew Fustini <drew@beagleboard.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/linux-omap/3139716.CMS8C0sQ7x@zen.local/
Link: https://lore.kernel.org/r/20200919200836.3218536-1-drew@beagleboard.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>